Alumina-boron carbide ceramics and methods of making and using the same

ABSTRACT

A ceramic body, as well as a method of making the ceramic body, wherein in one aspect a hot-pressing method is used to produce the ceramic body. In another aspect, a sintering to full density method is used to produce the ceramic body. The hot-pressed ceramic body contains between about 15 volume percent and about 35 volume percent of a boron carbide phase and at least about 50 volume percent of alumina, and the substrate has a fracture toughness (K IC , 18.5 Kg Load E&amp;C) greater than or equal to about 4.5 MPa·m 0.5 . The sintered to full density ceramic body contains between about 15 volume percent and about 50 volume percent of a boron carbide irregular-shaped phase and at least about 50 volume percent alumina. The substrate further includes at least one of the following components (a) through (b) wherein the sum of the amounts of components (a) and (b) is equal to an effective amount to achieve full densification wherein components (a) and (b) include (a) a metal component including one or more of aluminum, magnesium and zinc; and (b) a reduction component containing carbon.

CROSS-REFERENCE TO EARLIER APPLICATION

This patent application is a continuation-in-part to copending U.S.patent application Ser. No. 11/063,480 filed on Feb. 23, 2005 forALUMINA-BORON CARBIDE CERAMICS AND METHODS OF MAKING AND USING THE SAMEby Wu and Yeckley and assigned to Kennametal Inc. of Latrobe, Pa. 15650.

BACKGROUND OF THE INVENTION

The disclosure of the present patent application pertains to a ceramicbody that contains alumina and boron carbide, as well as a method ofmaking the same and a method of using the same. More specifically, thedisclosure of the present patent application pertains to a ceramic body(for use as a ceramic cutting insert or a substrate for a coated ceramiccutting insert or a ceramic wear part) that contains alumina and a boroncarbide phase, as well as a method (e.g., hot pressing method or apressureless sintering-HIPping method) of making the same and a methodof using the same.

Ceramic materials have been used as cutting inserts and as wear membersfor a number of years. These ceramic materials include silicon nitrideor silicon nitride-based ceramics, SiAlON or SiAlON-based ceramics, andalumina or alumina-based ceramics. One of the first ceramic cuttinginserts was an alumina cutting insert. See Dörre et al., “Alumina,Processing, Properties, and Applications”, Springer-Verlag (1984), pages254-265. The alumina cutting insert was essentially over 99.7 percentalumina. Later on, the alumina ceramic was modified by the addition oftitanium carbide. See Whitney, “Modern Ceramic Cutting Tool Materials”,Presentation at October, 1982 ASM Metals Congress in St. Louis, Mo.

Over the passage of time, there have been a number of other additivesused in conjunction with alumina to form an alumina-based ceramiccutting insert. Examples of the additives include the use of siliconcarbide whiskers such as the ceramics that appear to be disclosed in theU.S. Pat. No. 4,789,277 to Rhodes et al. and U.S. Pat. No. 4,961,757 toRhodes et al. In an alumina-SiC whisker ceramic, the Rhodes et al.patents appears to show that the (K_(IC)) fracture toughness increased(4.15 to 8.9 MPa·m^(0.5)) as the SiC whisker content increased from 0 to24 volume percent. The Rhodes et al. patents then appear to show thatthe fracture toughness decreased (8.9 to 7.6 MPa·m^(0.5)) as the SiCwhisker content increased from 24 to 35 volume percent. European PatentNo. 0 335 602 B1 to Lauder appears to disclose the use of siliconcarbide whiskers in alumina along with the addition of additives likezirconia, yttria, hafnia, magnesia, lanthana or other rare earth oxides,silicon nitride, titanium carbide, titanium nitride or mixtures thereof.The use of silicon carbide whiskers along with alumina is described inBillman et al., “Machining with Al₂O₃—SiC Whisker Cutting Tools”,Ceramic Bulletin, Vol. 67, No. 6 (1988) pages 1016-1019. U.S. Pat. No.4,343,909 to Adams et al. appears to disclose the use of zirconia andtitanium diboride along with alumina (and a sintering aid). U.S. Pat.No. 4,543,343 to Iyori et al. discloses the use of titanium boride andzirconia along with alumina.

In the article written by Liu and Ownby (Liu et al. entitled “PhysicalProperties of Alumina-Boron Carbide Whisker/Particle Composites” CeramicEng. Sci. Proc. 12 (7-8) pp. 1245-1253 (1991) there is a disclosure of aceramic comprising alumina and boron carbide particles. In this regard,the Liu et al. composites appear to disclose alumina (A16SG fromAlcoa)-boron carbide particle (0.2 to 7 μm particles size) compositesalong with boron carbide that is present in amounts of 5.0, 10.0, 15.0and 20.0 volume percent (the balance equals alumina). The examples wereeither sintered at 1500° C. or 1600° C. for 3 hours or hot-pressed underthe hot pressing parameters that comprised a temperature equal to 1520°C. for a duration equal to 20 minutes. The sintered composites had adensity less than 80 percent of the theoretical density. The hot pressedceramics had a density of greater than 98 percent of the theoreticaldensity. The hot pressing pressure seems to be absent from thedisclosures of this Liu et al. article.

This Liu et al. article appears to show that the fracture toughness(measured by the Chevron Notched Short Rod (CNSR) technique) improvesfrom 0 volume percent boron carbide particles to 5.0 volume percentboron carbide particles wherein the fracture toughness of the 5.0 volumepercent boron carbide particle-alumina ceramic equals about 5.2MPa·m^(0.5). However, the fracture toughness drops off at boron carbideparticle contents greater than 5.0 volume percent. More specifically,the fracture toughness diminishes at boron carbide particle contents of10.0, 15.0 and 20.0 volume percent. The fracture toughness of the 20.0volume percent boron carbide particle-alumina ceramic appears to equalabout 4.5 MPa·m^(0.5). Liu et al also shows that the flexural strengthimproves from 0 volume percent boron carbide particles to 5.0 volumepercent boron carbide particles. The 5.0 volume percent boron carbideparticle-alumina material has a flexural strength equal to about 575MPa. The flexural strength levels off (i.e., remains essentially thesame) at boron carbide particle contents greater than 5.0 volume percent(i.e., boron carbide particle contents of 10.0, 15.0 and 20.0 volumepercent). The 20.0 volume percent boron carbide particle-aluminamaterial has a flexural strength equal to about 590 MPa.

In the article (1991—American Institute of Physics) written by Liu etal. entitled “Boron Containing Ceramic Particulate and WhiskerEnhancement of the Fracture Toughness of Ceramic Matrix Composites”there is a disclosure of a ceramic comprising alumina and boron carbideparticles. These Liu et al composites appear to disclose α-alumina-boroncarbide particle composites wherein the boron carbide is present inamounts of 5.0, 10.0, 15.0 and 20.0 volume percent (the balance equalsalumina). The examples were hot-pressed under the hot pressingparameters that comprised a temperature equal to 1480° C. so that theceramic had a density of greater than 98 percent of the theoreticaldensity. The hot pressing duration and the hot pressing pressure appearto be absent from the disclosure of this Liu et al. article.

The Liu et al. articles show that the fracture toughness (CNSRtechnique) improves from 0 volume percent boron carbide particles to 5.0volume percent boron carbide particles wherein the fracture toughness ofthe 5.0 volume percent boron carbide particle-alumina ceramic equalsabout 5.5 MPa·m^(0.5). However, the fracture toughness drops off atboron carbide particle contents greater than 5.0 volume percent. Morespecifically, the fracture toughness diminishes at boron carbideparticle contents of 10.0, 15.0 and 20.0 volume percent. The fracturetoughness of the 20.0 volume percent boron carbide particle-aluminaceramic appears to equal about 4.6 MPa·m^(0.5).

In the article written by Liu et al. entitled “Boron Carbide ReinforcedAlumina Composites” Journal American Ceramic Society 74 (3) pp. 674-677(1991)) there is a disclosure of a ceramic comprising alumina and boroncarbide particles. The Liu et al. composites appear to disclose fineα-alumina (A16SG from Alcoa)-boron carbide “shard like” particle (0.2 to7 μm particles size) composites along with boron carbide that is presentin amounts of 5.0, 10.0, 15.0 and 20.0 volume percent (the balanceequals alumina). The examples were hot-pressed under the hot pressingparameters that comprised a temperature equal to 1520° C. for durationequal to 20 minutes so that the ceramic had a density of greater than 98percent of the theoretical density. The hot pressing pressure seems tobe absent from the disclosures of the Liu et al. articles.

This Liu et al. article appears to show that the fracture toughness(CNSR technique) improves from 0 volume percent boron carbide particlesto 5.0 volume percent boron carbide particles wherein the fracturetoughness of the 5.0 volume percent boron carbide particle-aluminaceramic equals about 5.3 MPa·m^(0.5). However, the fracture toughnessdrops off at boron carbide particle contents greater than 5.0 volumepercent. More specifically, the fracture toughness diminishes at boroncarbide particle contents of 10.0, 15.0 and 20.0 volume percent. Thefracture toughness of the 20.0 volume percent boron carbideparticle-alumina ceramic appears to equal about 4.6 MPa·m^(0.5). Liu etal also shows that the flexural strength improves from 0 volume percentboron carbide particles to 5.0 volume percent boron carbide particles.The 5.0 volume percent boron carbide particle-alumina material has aflexural strength equal to about 580 MPa. The flexural strength levelsoff (i.e., remains essentially the same) at boron carbide particlecontents greater than 5.0 volume percent (i.e., boron carbide particlecontents of 10.0, 15.0 and 20.0 volume percent). The 20.0 volume percentboron carbide particle-alumina material has a flexural strength equal toabout 600 MPa.

The Jung and Kim article entitled “Sintering and Characterization ofAl₂O₃—B₄C composites”, Journal of Material Science 26 (1991) pp.5037-5040 concerns the sintering of alumina-boron carbide composites.According to the article, for composites sintered at 1850° for 60minutes the density was about 97 percent for a boron carbide contentthat ranged between 5 to 20 volume percent boron carbide. According tothe Jung et al. article, the flexural strength had a maximum value of550 MPa for an alumina-20 volume percent boron carbide composite thathad been sintered at 1850° for 60 minutes. According to the Jung et al.article, for a composite sintered at 1850° for 60 minutes. The Vickersmicro-hardness increased with increasing boron carbide content to 30volume percent. For this same composite, the fracture toughness slightlyincreases with increasing boron carbide contents up to 20 volumepercent. The maximum fracture toughness is 4 MPa·m^(1/2).

In the article entitled “Microstructural Coarsening During Sintering ofBoron Carbide” by Dole et al. (J. Am. Ceram. Soc. 72 (6) pages 958-966(1989), it was reported that pressureless sintering of boron carbide at2300° C. produced only limited densification due to microstructuralcoarsening. According to the article entitled “Pressureless Sintering ofBoron Carbide” by Lee et al., J. Am. Ceram. Soc. 86(9) pages 1468-1473(2003), improved densities for pressureless sintered boron carbidebodies were obtained via rapid heating to liquid phase sinteringtemperatures so as to shorten the time for coarsening to occur. Further,the addition of carbon apparently caused a reaction with the boron oxide(B₂O₃) coating so as to improve densification. According to the articleentitled “Sintering of Boron Carbide Heat-Treated with Hydrogen” by Leeet al., J. Am. Ceram. Soc. 85(8) pages 2131-2133 (2002), hydrogen gas inthe sintering atmosphere was used to extract the boron oxide coating onthe boron carbide particles, and as a result, the process achieved apressureless sintered boron carbide body that exhibited a theoreticaldensity equal to 94.7 percent.

In the article by Dongsheng et al. entitled “Control of Boron ContentDuring Sintering of B₄C-dispersed Al₂O₃ Pellets” in Journal of CentralSouth Institute of Mining and Metallurgy (October 1994) carbon is addedto alumina (Al₂O₃) and boron carbide (B₄C) to reduce the loss of boronduring sintering, and thereby achieve precise control of the boroncontent in B₄C—Al₂O₃ pellets. According to this article, these B₄C—Al₂O₃are used in nuclear reactor cores and the boron content in these pelletsaffects the magnitude of neutron flux in the core. The article statesthat the reason the carbon controls the boron content is thatmicroparticles of carbon contained in the pellets oxidize before theB₄C, and can even reduce and carbonize the B₂O₃ to B₄C.

The article by Donsheng et al. entitled “Sintering behavior ofB₄C-dispersed Al₂O₃ Pellets” in Journal of Central South Institute ofMining and Metallurgy (February 1989) also concerned Al₂O₃—B₄C pelletsused in nuclear reactors. This article looked at the impact of particlesizes, as well as other factors, on properties of the sintered pellets.

Air Force Report AFML-TR-69-50 by E. Dow Whitney entitled “New andImproved Cutting Tool Materials” (1969) discloses an alumina-boroncarbide composite. At page 119, the Report reads:

-   -   The metal carbides, WC, TaC, TiC, B₄C and SiC were selected as        additives for improving the general properties of hot presses        alumina. Mixtures of Al₂O₃ containing 1.25 wt. % of each        additive were hot pressed at 1600° C., 2600 psi, for 30 minutes        in a nitrogen atmosphere. In FIGS. 147 to 149 are shown the        heating densification curves of these systems. Density increased        rapidly from about 1200° C. and reached almost 100% relative        density at temperatures below 1600° C.        Table 52 of the Air Force Report appears to show that the        addition of 1.25 weight percent boron carbide to alumina        increased the MOR from 30,700 psi (for alumina) to 42,500 psi        (alumina+1.25 weight percent boron carbide), but the hardness        decreased from 94.2 (R_(N)15) to 93.7 (R_(N)15).

U.S. Pat. No. 5,271,758 to Buljan et al. pertains to an alumina-basedcomposite that can include boron carbide and a Ni—Al metallic phase.Example 20 comprises” 8 v/o (Ni,Al), 27.6 v/o B₄C and 64.4 v/o Al₂O₃.U.S. '758 does not appear to specifically recite a hot pressing processfor Example 20. PCT Patent Publication WO 92/07102 to Buljan et al.published Apr. 30, 1992) appears to be related to U.S. '758. U.S. Pat.No. 5,279,191 to Buljan appears to disclose an alumina-based ceramicthat may include boron carbide. U.S. '191 requires the use of SiCreinforcement and a Ni—Al metal phase.

U.S. Pat. No. 5,162,270 to Ownby et al. pertains to an alumina ceramicthat has boron carbide whisker reinforcement. FIG. 1 appears to showspecific compositions in which the boron carbide whiskers appear tocomprise 0, 5.0, 10.0, 15.0, 20.0 and 30.0 volume percent of thecomposite (the balance alumina). These samples were hot pressed at 1520°C. under a pressure equal to 7500 psi to achieve a density equal togreater than abut 98 percent of theoretical density. The maximumfracture toughness (about 7.1 MPa·m^(0.5)) occurs at 15.0 volume percentboron carbide whiskers. There is a slight decrease in the fracturetoughness (about 7.1 MPa·m^(0.5) to about 7.0 MPa·m^(0.5)) when boroncarbide whisker content exceeds 15 volume percent. U.S. Pat. No.5,398,858 to Dugan et al. mentions the use of boron carbide whiskers toreinforce alumina. The specific application for the ceramic is in aroller guide.

The article by Liu and Ownby entitled “Densification of B₄C WhiskerReinforced Al₂O₃ Matrix Composites”, Proceedings of the First ChinaInternational Conference on High-Performance Ceramics (October, 1998,Beijing) pp. 415-419 pertains to the sintering of boron carbidewhisker-alumina composites. The boron carbide whisker contents were (involume percent): 0, 5, 10, 15, 20, 25, 30, 35 and 40.

The article by Liu et al. entitled “Enhanced Mechanical Properties ofAlumina by Dispersed Titanium Diboride Particulate Inclusions”, JournalAmerican Ceramic Society 74(1) pp. 241-243 (1991) discloses the use oftitanium diboride particles to improve mechanical properties of alumina.FIG. 2 shows the impact of the boron carbide particle content in analumina-based ceramic on the flexural strength wherein the boron carbidecontent ranges from 0 to 20.0 volume percent. Like in the other articlesto Liu et al., the flexural strength appears to level off (or remainsteady) for boron carbide contents that exceed 5.0 volume percent.

U.S. Pat. No. 4,745,091 to Landingham discloses an alumina-based ceramicthat has a nitride modifier (e.g. AlN or Si₃N₄) and dispersionparticles. A listing of the dispersion particles mentions boron carbide.According to the '091 patent, the nitride modifier can range from 0.1 to15.0 weight percent, and the dispersion particles can range between 0.1and 40.0 weight percent. There do not appear to be any actual examplesthat use boron carbide as dispersion particles.

U.S. Pat. No. 6,417,126 B1 to Yang discloses an alumina-based compositewith a boride (e.g., boron carbide) and metal carbide (e.g., siliconcarbide). The examples appear to disclose compositions comprisingalumina, silicon carbide, and boron carbide wherein the boron carbideranges between 0.5 and 5.4 weight percent. U.S. '126 appears to disclosethat the principal use of the ceramic is an industrial blast nozzle.U.S. Patent Application Publication US2002/0195752 A2 to Yang appears tobe related to U.S. '126. European Patent 0 208 910 to Suzuki et al.appears to disclose the use of boron carbide along with SiC whiskers inan alumina composite.

U.S. Pat. No. 5,164,345 to Rice et al. relates to an alumina-boroncarbide-silicon carbide composite. The end product is the result ofheating silicon dioxide, boron oxide, aluminum and carbon.

The article by Sato et al., “Sintering and Fracture Behavior ofComposites Based on Alumina-Zirconia (Yttria)—Nonoxides”, Journal dePhysique, Colloque C1, Supplement No. 2, Tome 47, February 1986 pp.C1-733 through C1-737 pertains to the sintering of alumina-containingcomposites including an alumina-zirconia-boron carbide composite. Table1 of Sato et al. shows various properties of a 50 volume percentAl₂O₃—40 volume percent ZrO₂ (no yttria)—10 volume B₄C composite, and an80 volume percent Al₂O₃—10 volume percent ZrO₂ (no yttria)—10 volumepercent B₄C composite. Each composite was hot pressed at 1500° C. and 2GPa for a duration of 30 minutes.

The article by Becher entitled “Microstructural Design of ToughenedCeramics” Journal American Ceramic Society 74(2) pp. 255-269 (1991)discusses toughening mechanisms. The principal toughening mechanism iscrack-bridging. Additives include silicon carbide whiskers, tetragonalzirconia and monoclinic zirconia.

U.S. Pat. No. 4,474,728 to Radford and U.S. Pat. No. 4,826,630 toRadford each discloses pellets that comprise alumina and boron carbide.These pellets appear to be useful as neutron absorbers.

While there have been ceramic bodies that comprise alumina and boroncarbide, there remains a need to provide an improved ceramic body thatcontains alumina and a boron carbide phase. There also remains the needto provide a method of making, as well as a method of using such animproved ceramic body that contains alumina and boron carbide. Further,there also remains the need to provide such a ceramic body of aluminaand boron carbide that exhibits properties that are especially usefulfor metalcutting. In addition, there remains a need to provide a methodof making (and a method of using) such a ceramic body of alumina andboron carbide that exhibits properties especially useful for metalcutting.

Exemplary of these properties is the ability of the ceramic body tomaintain its hardness even at higher operating temperatures, especiallythose temperatures associated with higher cutting speeds. Anotherexemplary property is the ability of the ceramic body to exhibit goodchemical resistance with respect to the workpiece material even at highoperating temperatures, especially those associated with higher cuttingspeeds.

Each one of these properties by itself, and especially when combinedtogether, provide for a ceramic body that is particularly useful as aceramic cutting insert for applications at higher cutting speeds whereinthere are generated higher operating temperatures. For example, a highercutting speed contemplated by applicants for ductile cast iron could bea speed equal to or greater than about 1500 surface feet per minute(about 457 surface meters per minute), and more preferably, a highercutting speed equal to or greater than about 2000 surface feet perminute (610 surface meters per minute).

We have found that hot pressing is one method that has been used to makethe above ceramic body containing alumina and boron carbide. Hotpressing has produced ceramic bodies, which contain alumina and boroncarbide, that exhibit acceptable properties including properties thatmake the ceramic body particularly useful for metalcutting.

While the hot-pressing process has produced an acceptable ceramicmaterial, the hot-pressing process typically has experienced drawbacks.One of these drawbacks is associated with the high cost to perform thehot-pressing process. Such a high cost increases the overall cost toproduce the ceramic body, and especially a ceramic used as ametalcutting insert. Another of these drawbacks was the inability of thehot-pressing process to permit the cost-effective fabrication of partsthat presented a complicated or complex shape or geometry. Such hotpressing is thus not cost-effective for the fabrication of parts with acomplicated or complex shape. It would be advantageous if the scope ofapplicable products for the alumina-boron carbide ceramic would be suchto include being fabricated by cost-effective methods.

Thus, another process to densify a powder compact would be sinteringincluding pressureless sintering. Pressureless sintering would be a moredesirable process as compared to hot-pressing because of the lower costassociated therewith, as well as the ability of the pressurelesssintering process to fabricate in a cost-effective fashion parts with acomplicated or complex shape or geometry. However, heretofore, thepressureless sintering of alumina-boron carbide powder compacts haddifficulty in achieving closed porosity due to the loss of boron duringsintering.

It is apparent that it would be highly desirable to provide analumina-boron carbide ceramic and a process of making the same that isnot as expensive to perform as compared to hot-pressing. If such aprocess were available, there would be expected to be a decrease in theoverall cost to produce the ceramic body.

It is also apparent that it would be highly desirable to provide analumina-boron carbide ceramic and a process of making the same whereinthe process provides for the ability to fabricate in a cost-effectivefashion parts with a complicated or complex shape or geometry. If such aprocess were available, there would be an increase in the scope ofapplicable products for the alumina-boron carbide ceramic.

It becomes apparent that it would be highly desirable to provide asintered alumina-boron carbide ceramic body, as well as a sinteringprocess to make an alumina-boron carbide ceramic body that exhibitsacceptable properties including acceptable properties that make itsuitable for use as a metalcutting insert.

SUMMARY OF THE INVENTION

In one form, the invention is a ceramic metalcutting insert for chipforming machining made from a starting powder mixture. The ceramicinsert body comprises a substrate that has a rake surface and a flanksurface wherein the rake surface and the flank surface intersect to forma cutting edge. The substrate comprises between about 15 volume percentand about 35 volume percent of a boron carbide irregular-shaped phaseand at least about 50 volume percent of alumina. The substrate has afracture toughness (K_(IC)) (18.5 Kg Load E&C) equal to or greater thanabout 4.5 MPa·m^(0.5).

In still another form the invention is a process for making a ceramic(wherein the ceramic has a preferred application for cutting toolapplications) comprising the steps of: providing a starting powdermixture that comprises between about 15 volume percent and about 35volume percent of a boron carbide powder, and at least about 50 volumepercent of alumina powder and no more than about 5 volume percent of asintering aid; consolidating the powder mixture at a temperature equalto between about 1400 degrees Centigrade and about 1850 degreesCentigrade to achieve a ceramic with a density equal to or greater than99 percent of the theoretical density.

In yet another form thereof, the invention is a ceramic body thatcomprises between about 15 volume percent and about 35 volume percent ofa boron carbide irregular-shaped phase, and at least about 50 volumepercent of alumina. The substrate has a fracture toughness (K_(IC))(18.5 Kg Load E&C) equal to or greater than about 4.5 MPa·m^(0.5).

In still another form thereof, the invention is a method of machining aworkpiece comprising the steps of: providing a workpiece; providing aceramic cutting insert having a rake surface and a flank surface whereinthe rake surface and the flank surface intersect to form a cutting edgeand the ceramic cutting insert having a substrate that comprises betweenabout 15 volume percent and about 35 volume percent of a boron carbidephase and at least about 50 volume percent alumina and has a fracturetoughness (K_(IC), 18.5 Kg Load E&C) greater than or equal to about 4.5MPa·m^(0.5); causing relative rotational movement between the workpieceand the ceramic cutting insert wherein the surface speed of the relativerotational movement is equal to or greater than about 457 surface metersper minute; and bringing the ceramic cutting insert and the workpieceinto contact with each other so as to remove material from theworkpiece.

In still another form thereof, the invention is a ceramic metalcuttinginsert for chip forming machining made from a starting powder mixture.The ceramic insert body includes a substrate that has a rake surface anda flank surface wherein the rake surface and the flank surface intersectto form a cutting edge. The substrate has a composition that containsbetween about 15 volume percent and about 50 volume percent of a boroncarbide irregular-shaped phase and at least about 50 volume percentalumina. The composition of the substrate further contains at least oneof the following components (a) through (b) wherein the sum of theamounts of components (a) and (b) is equal to an effective amount toachieve full densification and wherein components (a) and (b) comprise:(a) a metal component including one or more of aluminum, magnesium andzinc; or (b) a reduction component containing carbon.

In another form thereof, the invention is a ceramic metalcutting insertfor chip forming machining made from a starting powder mixture whereinthe ceramic insert body includes a substrate that has a rake surface anda flank surface wherein the rake surface and the flank surface intersectto form a cutting edge. The substrate has a composition that containsbetween about 15 volume percent and about 50 volume percent of a boroncarbide irregular-shaped phase and at least about 50 volume percentalumina. The substrate further contains at least one of the followingcomponents (a) through (c) wherein the sum of the amounts of components(a) and (b) and (c) is equal to an effective amount to achieve fulldensification and wherein components (a) and (b) and (c) comprise: (a) ametal component including one or more of aluminum; (b) a reductioncomponent containing carbon; or (c) an oxide including one or more ofzirconium oxide and hafnium oxide.

In still another form thereof, the invention is a method of making aceramic body comprising the steps of: providing a starting powdermixture comprising: between about 15 volume percent and about 50 volumepercent of a boron carbide irregular-shaped phase and at least about 50volume percent alumina, and the substrate further comprising at leastone of the following components (a) through (b) wherein the sum of theamounts of components (a) and (b) is equal to an effective amount toachieve full densification and wherein components (a) and (b) comprise:(a) a metal component including one or more of aluminum, magnesium andzinc; and (b) a reduction component containing carbon; and forming agreen compact from the starting powder mixture; sintering the greencompact to full density so as to form the ceramic body.

In yet another form thereof, the invention is a method of making aceramic body comprising the steps of: providing a starting powdermixture comprising: between about 15 volume percent and about 50 volumepercent of a boron carbide irregular-shaped phase and at least about 50volume percent alumina, and the substrate further comprising at leastone of the following components (a) through (c) wherein the sum of theamounts of components (a) and (b) and (c) is equal to an effectiveamount to achieve full densification wherein components (a) and (b) and(c) comprise: (a) a metal component including one or more of aluminum;(b) a reduction component including one or more of carbon; or (c) anoxide including one or more of zirconium oxide and hafnium oxide; andforming a green compact from the starting powder mixture: pressurelesssintering the green compact so as to form a sintered body; and hotisostatic pressing the sintered body so as to form the ceramic body.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings wherein thesedrawings form a part of this patent application:

FIG. 1 is an isometric view of a ceramic cutting insert that embodiesthe invention;

FIG. 2 is a colorized photomicrograph (50 micrometer scale) that showsthe microstructure of the ceramic body of Sample CA340-58 that has astarting composition of about 24.9 volume percent boron carbide powder,about 74.6 volume percent alumina powder, and sintering aid residue from0.5 volume percent of ytterbia (i.e., ytterbium oxide) as a sinteringaid in the starting powder mixture, and in the photomicrograph the lightphase is boron carbide;

FIG. 3. is a color photomicrograph (30 micrometer scale) that was madevia scanning electromicroscopy (SEM) techniques that shows themicrostructure of the ceramic body of Sample CA340-59 that has acomposition of about 25 volume percent boron carbide powder, about 74volume percent alumina powder and sintering aid residue from 1.0 volumepercent ytterbia (ytterbium oxide) as a sintering aid in the startingpowder mixture, and in the photomicrograph the dark phase is the boroncarbide and the light phase is a ytterbium-containing compound;

FIG. 4A is a colorized photograph (at a magnification equal to 30×) ofthe flank surface of a prior art ceramic cutting insert designatedherein as Comparative Insert #1 [KYON 3400] showing the nature of theflank wear on the cutting insert after completion (duration of 6minutes) of the testing set out in Table 5;

FIG. 4B is a colorized photograph (at a magnification equal to 30×) ofthe rake surface of a prior art ceramic cutting insert designated hereinas Comparative Insert #1 [KYON 3400] showing the nature of the craterwear on the cutting insert after completion (duration of 6 minutes) ofthe testing set out in Table 5;

FIG. 5A is a colorized photograph (at a magnification equal to 30×) ofthe flank surface of Sample CA340-58 showing the nature of the flankwear on the cutting insert after completion (duration of 6 minutes) ofthe testing set out in Table 5;

FIG. 5B is a colorized photograph (at a magnification equal to 30×) ofthe rake surface of Sample CA340-58 showing the nature of the craterwear on the cutting insert after completion (duration of 6 minutes) ofthe testing set out in Table 5;

FIG. 6 is a color optical photomicrograph (with a scale of 50micrometers) of the microstructure of Example CA340-80B;

FIG. 7 is an x-ray diffraction phase analysis for Example CA340-80Bwherein the peaks show the presence of aluminum oxide (Al₂O₃), boroncarbide (B₄C) and ytterbium boride (YbB₆);

FIG. 8 is a color optical photomicrograph (with a scale of 50micrometers) of the microstructure of Example CA340-80D;

FIG. 9 is an x-ray diffraction phase analysis for Example CA340-80Dwherein the peaks show the presence of aluminum oxide, boron carbide andytterbium boride;

FIG. 10 is a color optical photomicrograph (with a scale of 50micrometers) of the microstructure of Example CA340-87;

FIG. 11 is an x-ray diffraction phase analysis for Example CA340-87wherein the peaks show the presence of aluminum oxide (Al₂O₃), boroncarbide (B₄C) and zirconium boride (ZrB₂);

FIG. 12 is an x-ray diffraction phase analysis for Example CA340-89Awherein the peaks show the presence of aluminum oxide (Al₂O₃), boroncarbide (B₄C) and ytterbium boride (YbB₆); and

FIG. 13 is an x-ray diffraction phase analysis for Example CA340-76Awherein the peaks show the presence of aluminum oxide (Al₂O₃), boroncarbide (B₄C) and ytterbium boride (YbB₆).

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, FIG. 1 shows an embodiment of an indexableceramic cutting insert, i.e., a ceramic body, generally designated as20. Ceramic cutting insert 20 comprises a substrate 21 that has a rakesurface 22 and flank surfaces 24 wherein a cutting edge 26 is at theintersection of the rake surface 22 and the flank surfaces 24.

Throughout this description, selected physical properties of the ceramicbody (or ceramic substrate) are set forth. In regard to the method todetermine these properties, the fracture toughness (K_(IC)) (E&C) isdetermined by the method set forth in Evans & Charles, “FractureToughness Determination by Indentation”, J. American Ceramic Society,Vol. 59, Nos. 7-8, pages 371-372 using an 18.5 kilogram load. TheYoung's Modulus is determined by ASTM Standard E111-97 Standard TestMethod for Young's Modulus, Tangent Modulus and Chord Modulus. TheVicker's micro-hardness is determined by ASTM Standard E384-99e1Standard Test Method for Microindentation Hardness of Materials using an18.5 kilogram load.

As will become apparent from the description hereinafter, one groupingof specific examples uses hot pressing as the technique to fabricate thesubstrate of the ceramic body. Another grouping of specific examplesuses pressureless sintering followed by hot isostatic pressing (HIPing)to fabricate the substrate of the ceramic body. Although some propertiesor characteristics of the ceramic body may be discussed in connectionwith a ceramic body made either by hot pressing or by pressurelesssintering-HIPing, such discussion should not necessarily be consideredto be limited to a ceramic body made by a specific process.Compositional ranges and other properties discussed in reference to aceramic body produced by one specific process should not necessarily berestricted to that process, but may be applicable to a ceramic bodyproduced by another process.

The ceramic substrate (or ceramic body) of the ceramic cutting insertproduced by hot pressing has a composition that comprises primarilyalumina and a boron carbide irregular-shaped phase along with optionallylesser amounts of additives such as, for example, sintering aid residuefrom an addition of sintering aid to the starting powder mixture. Thesintering aid typically comprises in its broader range between about0.05 volume percent and about 5 volume percent of the starting powdermixture. A preferable range for the sintering aid is between about 0.1volume percent and about 1.5 volume percent of the starting powdermixture. A more preferable amount of sintering aid in the startingpowder mixture is about 0.5 volume percent of the starting powdermixture. For the ceramic substrate, the content (in volume percent) ofthe alumina in the ceramic is greater than the content (in volumepercent) volume percent of the boron carbide irregular-shaped phase inthe ceramic. The content (in volume percent) of the boron carbideirregular-shaped phase in the ceramic is greater than any othercomponent, except for the alumina in the ceramic.

Applicants contemplate that other additives may be added in amountseffective to improve the metalcutting performance characteristics (ofthe inventive ceramic cutting inserts) without undergoing a significantreaction with the boron carbide in the ceramic during the densificationof the ceramic body. In this regard, these additives include one or moreof the oxides of zirconium and/or hafnium and ceramic whiskersincluding, for example, silicon carbide whiskers, titanium carbidewhiskers, titanium nitride whiskers and titanium carbonitride whiskers.

In one compositional range, the substrate produced via a hot pressingprocess comprises between about 15 volume percent and about 35 volumepercent of a boron carbide irregular-shaped phase and at least about 50volume percent alumina and sintering aid residue. In anothercompositional range for the ceramic substrate produced via a hotpressing process, the substrate of the ceramic cutting insert comprisesbetween about 15 volume percent and about 35 volume percent of a boroncarbide irregular-shaped phase and between about 65 volume percent andabout 85 volume percent alumina and sintering aid residue. In yetanother compositional range for the ceramic substrate produced via a hotpressing process, the substrate of the ceramic cutting insert comprisesbetween about 20 volume percent and about 30 volume percent of a boroncarbide irregular-shaped phase and between about 70 volume percent andabout 80 volume percent alumina and sintering aid residue. In stillanother composition of the ceramic substrate produced via a hot pressingprocess, the substrate comprises about 25 volume percent of a boroncarbide irregular-shaped phase and about 75 volume percent alumina andsintering aid residue.

The hot-pressed ceramic substrate (or ceramic body) of the ceramiccutting insert exhibits certain physical properties. These physicalproperties include Young's Modulus (E), fracture toughness (K_(IC)) andVicker's microhardness. Values for these properties are set forthhereinafter.

In one embodiment of the hot-pressed ceramic body, the fracturetoughness (K_(IC), 18.5 kg load Evans & Charles) is greater than orequal to about 4.5 MPa·m^(1/2). In another embodiment of the hot-pressedceramic body, the fracture toughness (K_(IC), 18.5 kg load Evans &Charles) is greater than or equal to about 5.0 MPa·m^(1/2). In anotherembodiment of the hot-pressed ceramic body, the fracture toughness(K_(IC), 18.5 kg load Evans & Charles) is greater than or equal to about5.5 MPa·m^(1/2). In still another embodiment of the hot-pressed ceramicbody, the fracture toughness (K_(IC), 18.5 kg load Evans & Charles) isgreater than or equal to about 6.0 MPa·m^(1/2).

In one embodiment of the hot-pressed ceramic body, the Young's Modulus(ASTM Standard E111-97, Standard Test Method for Young's Modulus,Tangent Modulus and Chord Modulus) is greater than or equal to about 300GPa. In another embodiment of the hot-pressed ceramic body, the Young'sModulus is greater than or equal to about 350 GPa. In yet anotherembodiment of the hot-pressed ceramic body, the Young's Modulus isgreater than or equal to about 400 GPa.

In one embodiment of the hot-pressed ceramic body, the Vickersmicrohardness (ASTM Standard E384-99e1, Standard Test Method forMicroindentation Hardness of Materials, 18.5 kg load) is greater than orequal to about 17 GPa. In another embodiment of the hot-pressed ceramicbody, the Vickers microhardness is greater than or equal to about 18GPa. In yet another embodiment of the hot-pressed ceramic body, theVickers microhardness that is greater than or equal to about 19 GPa.

The ceramic body (after hot pressing) has a density that is greater thanor equal to about 3.6 grams per cubic centimeter. This equates to adensity that is greater than about 99.7 percent of the theoreticaldensity for a composition that comprises about 25 volume percent of aboron carbide irregular-shaped phase and about 75 volume percent aluminaand sintering aid residue.

Although the specific embodiment of FIG. 1 is a ceramic body that takeson the form of a substrate for an uncoated indexable ceramic cuttinginsert, applicants contemplate that the ceramic body has uses other thanas a substrate for a ceramic cutting insert. In this regard, the ceramicbody may have use as a substrate for a coated ceramic cutting insertincluding an indexable ceramic cutting insert. In addition, the ceramicbody may have use as a wear member. Exemplary wear members includenozzles for shot blasting and abrasive water jet applications.

As mentioned above, one useful technique to produce the ceramic body ishot pressing, and for some of the examples set forth below, hot pressingis the preferred process. However, as also mentioned above, applicantshave found that when certain additives are used, the process ofsintering to full density produces an acceptable ceramic body that has asatisfactory density, as well as other desirable properties. In thisregard, the preferable sintering to full density process comprisespressureless sintering followed by hot isostatic pressing (HIPing).

In general, the hot pressing process comprises the following steps thatare described hereinafter. The first step comprises providing a startingpowder mixture wherein the starting powder mixture has a compositionthat falls within one of the compositional ranges contemplated by theinvention as set forth in this patent application. The basic componentsof the starting powder mixture are a majority content of alumina powder,a minority content of boron carbide powder, and a minor content (e.g.,about 0.5 volume percent) of a sintering aid or in some cases, anotheradditive as mentioned above (e.g., the oxides of zirconium and/orhafnium and/or ceramic whiskers including, for example, silicon carbidewhiskers, titanium carbide whiskers, titanium nitride whiskers andtitanium carbonitride whiskers). The sintering aid can comprise one ormore materials that are suitable for use as a sintering aid forceramics. Exemplary sintering aids include oxides such as, for example,chromium oxide, yttrium oxide, yttrium aluminum garnet (YAG), ytterbiumoxide, lanthanum oxide and other rare earth oxides.

The second step in the hot pressing process comprises hot pressing thestarting powder mixture under pressure and heat to form the ceramicbody. The hot pressing conditions are generally defined by the hotpressing temperature, hot pressing pressure and the duration of the hotpressing process. In regard to the hot pressing parameters, the hotpressing temperature has one range that is between about 1400 degreesCentigrade and about 1850 degrees Centigrade, as well as a narrowerrange that is between about 1400 degrees Centigrade and about 1700degrees Centigrade. The hot pressing pressure has a range that isbetween about 20 MPa and about 50 MPa. The hot pressing duration has arange that is between about 20 minutes and about 90 minutes. The hotpressing process may occur under a vacuum (i.e., a pressure equal to orless than about 100 micrometers of mercury) or an inert gas atmosphere.

The hot pressing process produces a ceramic body that exhibits physicalproperties that include the fracture toughness (K_(IC) , 18.5 Kg LoadE&C), the Young's Modulus and the Vickers hardness. The typical valuesof these properties have been set forth in this patent application.

Tests to determine selected physical properties (e.g., Young's Modulus,Vickers microhardness and fracture toughness (K_(IC))), as well asmetalcutting performance, were conducted on specific examples (orsamples) of the ceramic body to compare the performance of specificexamples of the alumina-boron carbide irregular-shaped phase ceramiccutting inserts against the performance of standard cutting inserts. Thesteps of the process employed to make the alumina-boron carbideirregular-shaped phase ceramic cutting inserts that were the subjects ofthe tests to determine the physical properties and the metalcuttingperformance are set forth below.

In regard to the specific powders used in the samples, for most of theexamples, the boron carbide powder (which has a blocky-angular shape)was sold by Electro Abrasives (having a place of business at 701 WilletRoad, Buffalo N.Y. 14218) under the designation F800 wherein the powderhas the following properties: median particle size equal to about 15micrometers, a surfaces area (as measured by BET) equal to 1.5 m²/gram,an oxygen content equal to 0.64 weight percent, the total boroncontent=77.5 weight %, the total carbon content=21.5 weight %, iron=0.2weight %, and the total B+C content=98 weight %. Additional informationabout the boron carbide powders sold by Electro-Abrasives is availablethrough the website: http:/www.electroabrasives.com/b4C.html.

The other kind of boron carbide powder, which is designated as “HP” inTable 1, was sold under the designation Grade HP by H. C. Starck, Inc.45 Industrial Place, Newton, Mass. 02461. The Grade HP boron carbidepowder has the following chemical characteristics: a B:C ratio=3.8-3.9,minimum of 21.8 weight % carbon, maximum of 0.7 weight nitrogen, maximumof 1.0 weight % oxygen, maximum of 0.05 weight % iron, maximum of 0.15weight % of silicon, maximum of 0.05 weight percent aluminum, and amaximum of 0.5 weight % of other components. The Grade HP boron carbidepowder has the following physical characteristics: specific surface area(TRISTAR 3000 by BET per ASTM D 3663)=6 to 9 m²/gram; green density (10³kg/cm²)=1.5 to 1.7 g/cm²; particle size distribution with typical values(MASTERSIZER by Laser Light Diffraction per ASTM B 822, deglomerationwith high energy ultrasonic before analysis)=D90=6.5 micrometers,D50=2.5 micrometers, D10=0.4 micrometers. The above chemicalcharacteristics and physical characteristics are available from thewebsite http:/www.hcstarck.com and are set forth in the H. C. StarckData Sheet Number PD-4012.

The alumina powder was sold by Baikowski International (having a placeof business at 352 Westinghouse Blvd., Charlotte, N.C. 28273) under thedesignation SM8 wherein the powder has the following properties: BETspecific surface area equal to 10 m²/gram, an alpha crystal structure,an alpha crystallite size/XRD=50 nanometers (nm), an ultimate particlesize/TEM=400 nanometers (nm), and a purity greater than 99.99 percent.The agglomerate size distribution/sedigraph is: D20=0.2 micrometers;D50=0.3 micrometers; D90=0.7 micrometers. This information about the SM8alumina powder is available at the website: http:/www.baikowski.com.

Another kind of alumina powder was used for at least one other example,and that was alumina powder sold under the designation “HPA-0.5” bySasol North America, Inc., Ceralox Division, having a place of businessat 7800 South Kolb Road, Tucson, Ariz. 85706. The HPA-0.5 alumina powderhas the following properties: a purity equal to 99.99 weight percent; asurface area=9.0 m²/gram; a green density equal to 2.19 grams/cubiccentimeter; and a particle size distribution of D90=1.2 micrometers,D50=0.4 micrometers and D10=0.2 micrometers. Additional informationabout the properties and the HPA-0.5 alumina powder can be found at thewebsite: http:/www.ceralox.com/Documents/PDFfiles/TDS-ceramicpowders.pdf.

For the sintering aids, the yttrium oxide powder was sold by MolycorpInc. (having a place of business at 67750 Bailey Road, Mountain Pass,Calif. 97366) wherein the powder had the following properties: surfacearea equal to 1.8 m²/gram, a particle size (Microtrac d50) equal to 3-6micrometers (μm), and a purity equal to greater than 99.0 percent. Theytterbium oxide powder was sold by MolyCorp Inc. under the designationYb₂O₃ 99% and has the following properties: particle size (FAPS) 3 μmmax. and a purity greater than 99 percent. The lanthanum oxide powderwas sold by MolyCorp Inc. under the designation La₂O₃ 99.99% and has thefollowing properties: particle size (FAPS) 5-10 μm maximum and a puritygreater than 99.9 percent. The YAG powder was sold by Cerac Inc. (havinga place of business at P.O. Box 1178, Milwaukee, Wis. 53201-1178) underthe designation Y-2000 and has the following properties: formula isY₃Al₅O₁₂, average particle size −325 mesh and a purity greater than 99.9percent.

There are other powder components that were used in conjunction with thepressureless sintered ceramic bodies. These powder components will bedescribed in more detail hereinafter.

To produce the starting powder mixture, the mixture of the startingpowders of alumina and boron carbide and the sintering aid was subjectedto ball-milling using high purity alumina cycloids for a duration equalto about 36 hours in alcohol. After completion of the ball-milling, thepowder mixture was dried.

For all of the first grouping of specific examples, each one of thestarting powder mixture was hot pressed to form a ceramic body. For allof the first grouping of specific examples, unless indicated to thecontrary, the hot pressing was done using a graphite die and graphiterams, and the hot pressing parameters were a temperature equal to about1650 degrees Centigrade for a duration of about 1 hour under a pressureof about 35 MPa. The ceramic body was then finished ground to form thegeometries of the alumina-boron carbide irregular-shaped phase ceramiccutting inserts used in the metalcutting tests set forth below. Thegeometries of the ceramic cutting inserts are set forth in each of thetables.

Table 1 below sets forth the starting powder compositions for a numberof the compositions that are contained in the Tables set forthhereinafter. TABLE 1 Compositions of the Starting Powder Mixtures forthe Hot-Pressed Samples of the Ceramic Cutting Inserts as Reported inthe Tables Boron Carbide Hot-Pressed Alumina Particles Additive Sample(volume percent) (volume percent) (volume percent) CA340-30 74.6 (SM8)24.9 (F800) 0.5 yttria CA340-62 74.6 (SM8) 24.9 (HP) 0.5 yttria CA340-5774.6 (SM8) 24.9 (F800) 0.5 YAG CA340-58 74.6 (SM8) 24.9 (F800) 0.5ytterbia CA340-63 74.6 (SM8) 24.9 (F800) 0.5 lanthanum oxide CA340-6774.6 (SM8) 24.9 (F800) 0.5 ytterbia AA301-013 74.5 (HPA-0.5) 25 (F800)0.5 yttria CA340-59 74.25 (SM8) 24.75 (F800) 1.0 ytterbia

Table 1 sets forth herein above presents the compositions of the samplesof the hot-pressed ceramic cutting inserts that were subjected tometalcutting tests, and testing for physical properties, wherein thetest results are set forth in the Tables in this patent application. Thecompositions are reported in volume percent of the starting powdermixture. Except for Sample AA301-013, which used the Ceralox HPA-0.5alumina powder, all of these samples used the SM8 alumina powderdescribed earlier herein. The designation “F800” for the boron carbidepowder means that the boron carbide powder was the F800 boron carbidepowder from Electro-Abrasives described earlier herein. The designation“HP” for the boron carbide powder means the boron carbide powder was theHP boron carbide powder from H. C. Starck described earlier.

Table 2 set forth herein presents the results of measuring the Young'sModulus according to ASTM Standard E111-97 wherein the results arereported in gigapascals (GPa), the Vicker's Microhardness (18.5 kg load)according to ASTM Standard E384-99e1 wherein the results are reported ingigapascals (GPa), and the fracture toughness (K_(IC)) as measuredaccording to Evans & Charles using an 18.5 kg load and reported inMPa·m^(1/2). TABLE 2 Room Temperature Properties for SelectedHot-Pressed Compositions Vickers Hot-Pressed Young's Modulus −Microharadness Fracture Toughness Composition E (GPa) (VHN (GPa)(K_(IC)) (MPa · m^(1/2)) AA301-013 392 18.9 — CA340-30 395 18.7 5.46CA340-57 414 18.3 5.00 CA340-58 400 18.1 4.94 CA340-62 395 18.7 4.82CA340-63 399 18.1 5.09

FIG. 2 is a colorized photomicrograph that shows the microstructure ofthe hot-pressed ceramic body of Sample CA340-58 that has a startingcomposition set forth in Table 1 and the Room Temperature properties setforth in Table 2. Looking at the photomicrograph (FIG. 2), applicantsbelieve that four phases are shown in FIG. 2. What applicants believe tobe the boron carbide (B₄C) phase is shown in FIG. 2 as a bright phase.In regard to the ytterbium boride (YbB₆) phase, which applicants believeis present, while this phase is visible under a scanning electronmicroscope, the resolution in this optical photomicrograph (FIG. 2) issuch that this phase is difficult to distinguish. Applicants considerthe balance of the microstructure shown in FIG. 2 to comprise a matrix.Applicants believe that the matrix comprises an alumina (aluminum oxide)phase and an alumina-based solid solution phase wherein the contrast incolor distinguishes between these phases. In this regard, whatapplicants believe to be the alumina phase is shown in FIG. 2 as thephase of the matrix that is lighter (relative to the other matrix phase)in color, and what applicants believe to be the alumina-based solidsolution phase is shown in FIG. 2 as the phase of the matrix that isdarker (relative to the other matrix phase) in color.

FIG. 3. is a photomicrograph that was made via scanningelectromicroscopy (SEM) techniques that shows the microstructure of thehot-pressed ceramic body of Sample CA340-59 that has a composition asset forth in Table 1. Looking at the photomicrograph (FIG. 3),applicants believe that four phases are shown in FIG. 3. What applicantsbelieve to be the boron carbide (B₄C) phase is shown in FIG. 3 as a darkphase wherein the boron carbide presents an irregular shape. It can beseen that the boron carbide phase appears to be about 6 micrometers orless, and more typically about 3 micrometers or less, in its majordimension. What applicants believe to be a ytterbium-containing compound(possibly ytterbium boride (YbB₆)) phase is shown in FIG. 3 as thebright phase. Applicants consider the balance of the microstructureshown in FIG. 3 to comprise a matrix. Applicants believe that the matrixcomprises an alumina (aluminum oxide) phase and an alumina-based solidsolution phase wherein the contrast in color distinguishes between thesephases. In this regard, what applicants believe to be the alumina phaseis shown in FIG. 3 as the phase of the matrix that is lighter (relativeto the other matrix phase) in color, and what applicants believe to bethe alumina-based solid solution phase is shown in FIG. 3 as the phaseof the matrix that is darker (relative to the other matrix phase) incolor.

Table 3 set out below reports the results from turning a round clean barof ductile cast iron (80-55-06) wherein these results show a comparisonbetween a hot-pressed ceramic cutting insert of the invention(designated as Sample AA 301-013 wherein the starting powder compositionis set forth in Table 1) and a number of comparative cutting inserts.The inventive ceramic cutting insert (Sample AA 301-013) has acomposition of about 25 volume percent of a boron carbideirregular-shaped phase, about 74.5 volume percent alumina, and sinteringaid residue from about 0.5 volume percent of yttria sintering aid in thestarting powder mixture. Comparative Cutting Insert K090 has acomposition that comprises about 30 volume percent titanium carbide andthe balance (70 volume percent) alumina. Comparative Insert Kyon 1615has a composition that comprises 75 volume percent alumina-25 volumepercent titanium carbonitride. The cutting insert that carries thedesignation LA 17/02 in the tables has a composition of 75 volumepercent alumina-25 volume percent titanium carbonitride. The cuttinginsert that carries the designation CB347-216 has a composition of 42volume percent alumina-43 volume percent titanium carbonitride-15 volumepercent silicon carbide whiskers. The cutting insert that has thedesignation alumina-SiC whisker is a cutting insert that has acomposition of about 15 volume percent silicon carbide whiskers and thebalance (about 85 volume percent) alumina. Comparative Cutting InsertKYON 3400 is a chemical vapor deposition (CVD) coated silicon nitridesubstrate.

This metalcutting test comprised the turning of a round clean bar ofductile Cast Iron 80-55-06. The turning parameters were: a speed of 1500surface feet per minute (457 surface meters per minute), a feed of 0.015inches (0.38 millimeters) per revolution, and a depth of cut of 0.100inches (2.54 millimeters) d.o.c. The metalcutting was dry, i.e., nocoolant. The geometries of the cutting inserts are set forth in Table 3below wherein the lead angle for all of the cutting inserts was 15degrees. The geometries are identified according to the AmericanNational Standard for Cutting Tools-Indexable Inserts—IdentificationSystem, ANSI B212.4-1986. The failure criteria for this test is asfollows: Flank Wear (UNIF)=0.020 inches (0.508 mm); Flank Wear(MAX)=0.020 inches (0.508 mm); Nose Wear=0.020 inches (0.508 mm); andTrailing edge wear=0.020 inches (0.508 mm). TABLE 3 Metalcutting TestResults for Turning at a Speed of 1500 Surface Feet Per Minute ofDuctile Cast Iron 80-55-06 Using Different Cutting Inserts Rep. 1ToolRep. 2 Tool Rep. 3 Tool Mean T.L. Tool Material Geometry Life Life Life(minutes) AA301-013 SNG433T0425 6.6 3.4 5.1 5.0 KO90 SNG453T0820 0.8 0.3— 0.4 KYON 1615 SNG453T0820 0.3 0.7 — 0.3 LA 17/02 SNG454T0825 1.3 3.0 —1.4 CB347-216 SNG453T0425 2.0 3.3 3.0 2.8 Alumina-SiC SNG453T0820 4.74.1 2.0 3.6 Whisker KYON 3400 SNG453T0820 — 3.1 3.1 3.1

As can be seen from the insert designations set forth in the secondcolumn from the left side in Table 3, the sizes, geometries and edgepreparations for some of the cutting inserts were different. Based uponapplicants' experience, and later verified by additional tests, thesedifferences in sizes, geometries and edge preparations between thecutting inserts that were tested did not have a significant impact uponthe test results. Hence, applicants believe that the test resultsreported in Table 3 comprise a fair comparison between cutting insertsof the invention and the other cutting inserts.

These test results show that the hot-pressed ceramic cutting insert ofthe invention (Sample AA301-013) exhibited superior tool life whencutting at a speed equal to 1500 sfm (457 smm) as compared to a numberof other prior art ceramic cutting inserts. More specifically, theinvention showed excellent crater wear and nose wear resistance, ascompared with the comparative cutting inserts. The crater wear and nosewear resistance are the key factors for controlling the tool life whencutting (e.g., turning) at a high speed (e.g., a speed equal to 1500 sfm(457 smm)). In other words, better crater wear and nose wear propertiesresult in a longer tool life for a ceramic cutting insert when cutting(e.g., turning) at a high speed (e.g., a speed equal to 1500 sfm (457smm)).

Table 4 set out below reports the results from turning a round clean barof ductile cast iron (80-55-06) wherein the results show a comparisonbetween a hot-pressed ceramic cutting insert of the invention(designated as Sample CA340-67) and a comparative cutting insert(Kyon3400). Sample CA340-67 has a composition of about 24.9 volumepercent of the boron carbide irregular-shaped phase, about 74.6 volumepercent alumina, and sintering aid (ytterbia) residue that is from astarting powder content of sintering aid equal to about 0.5 volumepercent. The turning parameters were: a speed of 2000 surface feet perminute (609.6 surface meters per minute), a feed of 0.015 inches (0.38millimeters) per revolution, and a depth of cut of 0.100 inches (2.54millimeters) d.o.c. The metalcutting was dry, i.e., no coolant. Thegeometries of the cutting inserts are set forth in Table 4 below whereinthe lead angle for all of the cutting inserts was 15 degrees. Thefailure criteria for this test is as follows: Flank Wear (UNIF)=0.020inches (0.508 mm); Flank Wear (MAX)=0.020 inches (0.508 mm); NoseWear=0.020 inches (0.508 mm); and Trailing Edge Wear=0.020 inches (0.508mm). TABLE 4 Metalcutting Test Results (tool life in minutes) forTurning at a Speed of 2000 Surface Feet Per Minute of Ductile Cast Iron80-55-06 Tool Life Mean Tool Tool Material Geometry Rep 1 Rep 2 Life(minutes) Kyon 3400 SNGN433T0820 2.5 3.9 3.2 CA340-67 SNGN433T0820 8.07.7 7.8The inventive hot-pressed ceramic cutting insert (Sample CA340-67)significantly outperformed the cutting insert of the comparative grade(KYON 3400). Applicants believe that this improvement in performance wasdue to the superior chemical wear resistance provided by the inventiveceramic cutting inserts at higher cutting speeds (e.g., 2000 sfm (610smm)) wherein at such higher cutting speeds, the chemical wear exertsgreat influence over (i.e., dominates) the tool life.

Additional metalcutting test results demonstrate the performance ofspecific samples of the ceramic cutting insert of the invention. Thesetest results are set forth below.

Except for the speed, each one of the tests referred to in Tables 5 and6 was conducted at the following parameters: the feed equal to 0.015inches (0.381 millimeters); the Depth of cut (DOC) equal to 0.100 inches(2.54 mm); and the coolant: dry. The speed for the tests reported inTable 5 was 1500 feet per minute (457 meters per minute) and the speedfor the tests reported in Table 6 was 2000 feet per minute (610 metersper minute). For each of the tests, the geometry of the cutting insertwas a SNG433T0820 style of cutting insert that had a negative 5 degreelead angle. The workpiece material was a round clean bar of ductile castiron (80-55-06). The failure criteria for these tests set forth inTables 5 and 6 were as follows: Flank Wear (UNIF)=0.020 inches (0.508mm); Flank Wear (MAX)=0.020 inches (0.508 mm); Nose Wear=0.020 inches(0.508 mm); and Trailing Edge Wear=0.020 inches (0.508 mm). TABLE 5Metalcutting Test: Turning DCI 80-55-06 SNG-433T0820, 1500 sfm/.015ipr/.1″ doc/dry Average Insert Wear After 6 min. Turning Mean ToolInsert # FW MW NW TW Life (min) 1. KYON 3400 0.0124 0.0170 0.0190 0.01465.6 2. CA340-30 0.0143 0.0173 0.0165 0.0153 6.8 3. CA340-62 0.01390.0205 0.0154 0.0138 5.0 4. CA340-57 0.0130 0.0161 0.0148 0.0152 7.2 5.CA340-58 0.0129 0.0150 0.0152 0.0154 7.0 6. CA340-63 0.0125 0.01530.0153 0.0153 7.4In Table 5 above, the designations “FW” means average flank wearreported in inches, “MW” means average maximum flank wear reported ininches, “NW” means average nose wear reported in inches, and “TW” meansaverage trailing edge wear reported in inches. The mean tool life isreported in minutes.

Referring to the test results presented in Table 5 above, it is apparentthat, for the most part, the hot-pressed ceramic cutting inserts of theinvention outperformed the KYON 3400 ceramic cutting insert. The KYON3400 cutting insert is a commercial cutting insert that is well-acceptedfor the use in the turning of ductile cast iron. More specifically,except for Insert No. 3 (Sample CA340-62) which had a mean tool lifeequal to about 89.2 percent of the mean tool life of the KYON 3400cutting insert, all of the ceramic cutting inserts demonstrated animproved mean tool life. In this regard, Insert No. 2 (Sample CA340-30)had a mean tool life equal to about 121.4 percent of the mean tool lifeof the KYON 3400 cutting insert, Insert No. 4 (Sample CA340-57) had amean tool life equal to about 128.6 percent of the mean tool life of theKYON 3400 cutting insert, Insert No. 5 (Sample CA340-58) had a mean toollife equal to 125 percent of the mean tool life of the KYON 3400 cuttinginsert, and Insert No. 6 (Sample CA340-63), which used the lanthanumoxide sintering aid, had a mean tool life equal to about 132.1 percentof the mean tool life of the KYON 3400 cutting insert.

Based upon a comparison of the test results for Insert No. 2 and InsertNo. 3, it appears that the ceramic cutting insert that used the F800boron carbide (from Electro-Abrasives) had better results (i.e., alonger mean tool life) than the ceramic cutting insert that used the HPboron carbide (from H. C. Starck).

A comparison of the test results for an alumina-boron carbideirregular-shaped phase ceramic cutting insert using yttria as thesintering aid (i.e., Insert No. 2) against the alumina-boron carbideirregular-shaped phase ceramic cutting inserts using other sinteringaids shows that these other sintering aids (i.e., YAG, ytterbium andLa₂O₃) provided for improved results in the form of a longer mean toollife.

FIGS. 4A and 5A illustrate the comparison of the flank wear propertiesbetween a Comparative Insert #1 and a hot-pressed cutting insert of theinvention (Sample CA340-58). More specifically, FIG. 4A is a colorizedphotograph (at a magnification equal to 30×) of the flank surface of theComparative Insert #1 [KYON 3400] showing the nature of the flank wearon the cutting insert after completion (duration of 6 minutes) of thetesting set out in Table 5. FIG. 5A is a colorized photograph (at amagnification equal to 30×) of the flank surface of Sample CA340-58showing the nature of the flank wear on the cutting insert aftercompletion (duration of 6 minutes) of the testing set out in Table 5. Itis apparent from an examination of the cutting inserts shown in FIGS. 4Aand 5A, that the inventive hot-pressed ceramic cutting insertexperienced less flank wear and a more uniform flank wear than did theprior art comparative cutting insert (Comparative Insert #1).

FIGS. 4B and 5B show a comparison of the crater wear properties betweena Comparative Insert #1 and a hot-pressed cutting insert of theinvention (Sample CA340-58). More specifically, FIG. 4B is a colorizedphotograph (at a magnification equal to 30×) of the rake surface of theComparative Insert #1 [KYON 3400] showing the nature of the crater wearon the cutting insert after completion (duration of 6 minutes) of thetesting set out in Table 5. FIG. 5B is a colorized photograph (at amagnification equal to 30×) of the rake surface of Sample CA340-58showing the nature of the crater wear on the cutting insert aftercompletion (duration of 6 minutes) of the testing set out in Table 5. Itis apparent from an examination of FIGS. 4B and 5B that the inventivehot-pressed ceramic cutting insert (Sample CA340-58) experienced lesscrater wear than did the prior art cutting insert (Comparative Insert#1).

Metalcutting tests also show that the inventive hot-pressed ceramiccutting inserts exhibit better performance (i.e., tool life) at evenhigher cutting speeds, e.g., on the order of 2000 sfm (610 smm). In thisregard, Table 6 below sets forth the results for the turning of ductilecast iron at the parameters (including a speed equal to 2000 sfm (610smm)) set forth by cutting inserts of the geometry (SNG-433T0820)presented in Table 6. As seen by the results presented in Table 6, theinventive cutting inserts exhibit a much greater mean tool life than theKYON 3400 cutting insert. More specifically, the Insert No. 2 (SampleCA340-30), which is an alumina-boron carbide irregular-shaped phaseceramic cutting insert that used the F800 boron carbide, had a mean toollife equal to 244 percent of the mean tool life of the KYON 3400 ceramiccutting insert. Insert No. 5 (Sample CA340-58), which is analumina-boron carbide irregular-shaped phase ceramic cutting insert thatused ytterbia as the sintering aid, had a mean tool life that was 292percent of the mean tool life of the KYON 3400 ceramic cutting insert.TABLE 6 Mean Tool Life Reported in Minutes Turning DCI 80-55-06SNG-433T0820, 2000 sfm/.015 ipr/.1″ doc/dry Insert # Test No. 1 Test No.2 Mean Tool Life (min) 1. KYON 3400 3.0 1.9 2.5 2. CA340-30 7.4 4.7 6.15. CA340-58 7.5 7.0 7.3 6. CA340-63 4.5 6.6 5.5

When increasing the turning speeds from 1500 sfm (457 m/min) to 2000 sfm(610 m/min), as shown in metal cutting test T10919 (Table 6 below), theperformance of KY3400 degraded considerably while the influence ofspeeds on the performance of alumina-boron carbide composites was not sosignificant. The cutting insert (Sample CA340-63) that used lanthanumoxide as the sintering aid had a mean tool life that was over twice aslong (i.e., 5.5 minutes vs. 2.5 minutes) as the mean tool life for theKYON 3400 cutting insert.

The specific examples set forth hereinafter, unless specificallydesignated as being produced by a different process, were produced bythe process of sintering to full density wherein the preferred sinteringto full density process comprises first pressureless sintering and thenHIPing. To the extent that the starting powder mixtures used alumina,the alumina powder was the SM8 aluminum oxide sold by BaikowskiInternational. For the examples that used boron carbide, the boroncarbide powder was the F800 sold by Electro Abrasives. For the examplesthat used ytterbia, the ytterbium oxide was the same as that used by theearlier examples and sold by Molycorp Inc.

The specific examples produced by the sintering to full density processused some additional powders in the starting powder mixture. Thesepowders are described below.

The zirconium oxide powder was sold by TOSOH USA, INC (3600 Gantz Road,Grove City, Ohio 43123 USA, USA toll free: 866-844-6953 Fax:+1-614-875-8066, e-mail: info@tosohusa.com), under the designationTZ-6Y, and has the following properties: 6% mol % Y₂O₃ and SpecificSurface Area 16 m²/g.

The aluminum powder was sold by ATLANTIC EQUIPMENT ENGINEERS (13 FosterStreet, Bergenfield, N.J. 07621, Phone: (800) 486-2436 or (201) 384-5606FAX: 201-387-0291, http://www.micronmetals.com), under the designationAL-104, and has the following properties: spherical powder, 99.9%purity, PSD of 1-3 micron.

The carbon black is typically available from many sources, such ascarbon black Raven 410, supplied by Columbian Chemicals Company (1800West Oak Commons Court, Marietta, Ga. 30062-2253, Phone: 770-792-9400).The Raven 410 has an average particle size of 101 nm, NSA surface areaof 26 m²/g and 0.7% volatile.

Although none of the specific examples contained magnesium or zinc orhafnium oxide, applicants understand that suitable magnesium, zinc andhafnium oxide powders are available from ATLANTIC EQUIPMENT ENGINEERS(13 Foster Street, Bergenfield, N.J. 07621, Phone: (800) 486-2436 or(201) 384-5606 FAX: 201-387-0291, http://www.micronmetals.com),

To produce the starting powder mixture for the below examples, themixture of the starting powders of alumina and boron carbide and thesintering aid was subjected to ball-milling using high purity aluminacycloids for a duration equal to about 36 hours in alcohol. Aftercompletion of the ball-milling, the powder mixture was dried. Table 7below sets forth the composition (volume percent) of the starting powdermixture for the examples (i.e., Mix No.) listed in the table. TABLE 7Starting Powder Compositions (Volume Percent) for Examples alumi- ytter-num boron bium alumi- car- zirconium Mix No. oxide carbide oxide num bonoxide CA340-76 74.62 24.88 0.50 — — — CA340-80B 74.12 24.88 0.50 0.50 —— CA340-80D 74.37 24.88 0.50 — 0.25 — CA340-87 55.00 37.50 — — — 7.50CA340-89A 73.00 25.00 0.50 1.50 — — CA340-74A 67.00 30.00 0.50 2.50 — —CA340-89B 73.75 25.00 0.50 — 0.75 — CA340-76A 72.12 24.88 0.50 2.50 — —

Example CA340-76 was hot-pressed according to the following parameters:hot pressing temperature equal to 1550° C., a hot-pressing pressure (inargon) equal to 28 MPa, and a hot-pressing duration equal to 60 minutes.

Table 8 below sets forth the processing parameters for the inventiveexamples produced via a sintering to full density process and asidentified in Table 8. A ceramic body is considered to be at “fulldensity” when its density is equal to or greater than about 98% percentof the theoretical density, and preferably, equal to or greater thanabout 99% of the theoretical density, and wherein the theoreticaldensity is based upon the original starting powders and the originalconcentrations thereof.

The term “pressureless sintering” is a relative term in that it is usedto describe a densification process in a relative sense to otherdensification processes wherein these other densification processesdensify at higher pressures than the pressureless sintering process.Examples of these other densification processes, which employ higherpressures, include hot-pressing processes, gas pressure sinteringprocesses and hot isostatic pressing processes. The preferred conditionsfor the pressureless sintering of the specific examples set forth hereinwere an argon protective atmosphere that was held at a pressure ofbetween about 0.5 to about 1.0 pounds per square inch (psi), whichequates in metric units to between about 3.4 KPa and about 6.9 KPa.However, it should be appreciated that applicants intend that“pressureless” sintering include sintering processes that use a pressureequal to or less than one atmosphere (760 torr) in an inert or areducing gas atmosphere.

Furthermore, in the pressureless sintering process the preferred heatingrate to the sintering temperature was equal to about 75° C. per minuteand the preferred cooling rate from the sintering temperature to roomtemperature was equal to about 100° C. per minute. It should beappreciated that the heating rate can range between about 5° C. perminute and about 250° C. per minute, and the cooling rate can rangebetween about 5° C. per minute and about 250° C. per minute.

The pressureless sintering occurs at a temperature that is equal tobetween about 1500° C. and about 1850° C. for a duration equal tobetween about 15 minutes and about 180 minutes. As an alternative, thepressureless sintering can occur at a temperature equal to betewen about1600° C. and about 1750° C. for a duration equal to between about 30minutes and about 60 minutes.

In regard to the processing steps, after completion of the ball-millingstep, the starting powder mixture was pressed into a green SNG-433cutting insert blank compact and then sintered according to theparameters set forth below in Table 8 to form a sintered body. The greenbody was then cold isostatically pressed at a pressure equal to 30,000psi (206.9 MPa) to increase the green density thereof. Alternatively,the powder mixture could be pressed with a fugitive binder blendedtherein to avoid the need for cold isostatic pressing. Applicantscontemplate that in those cases that use the fugitive binder, thefugitive binder can supply all or a part of the carbon, and hence,function as the reduction component.

As mentioned above, it is preferable that the pressureless sintered bodyis then subjected to a hot isostatic pressing (HIP) step. ExemplaryHIPing parameters are set out in Table 8. For all of the examples thehot isostatic pressing step occurred in argon. TABLE 8 PressurelessSintering and HIP Parameters for Examples Sintering Sintering HIP HIPHIP Temp Duration Temp Pressure Duration Mix No. (° C.) (Minutes) (° C.)(MPa) (Minutes) CA340-80B 1850 60 1650 137 60 CA340-80D 1850 60 1650 13760 CA340-87 1825 60 1575 137 60 CA-340-89A 1825 60 1575 138 60CA-340-74A 1800 60 1575 138 60 CA-340-89B 1825 60 1575 138 60 CA-340-76A1825 60 1650 138 60

The HIPing can occur at a temperature equal to between about 1400° C.and about 1725° C., a pressure equal to or greater than about 3.4 MPa,and for a duration equal to between about 15 minutes and about 120minutes. As an alternative, the HIPing can occur at a temperature equalto between about 1600° C. and about 1700° C., a pressure equal to orgreater than about 50 MPa and for a duration equal to between about 30minutes and about 60 minutes.

Tables 9 through 15 present the results of taking three measurements forselected ceramic materials of selected physical properties per thetechniques identified above. These three measurements were averaged andthe average is set out. TABLE 9 Selected Physical Parameters forHot-Pressed Alumina-Boron Carbide Composition CA340-76 Young's VickersModulus Hardness K_(IC) (E & C) Sample (GPa) (GPa) (MPa · m^(1/2)) 1386.00 16.92 5.47 2 386.00 16.45 5.51 3 386.00 16.34 5.44 Average 386.0016.57 5.47

The density of Example CA340-76 was equal to 3.677 grams per cubiccentimeter, which is equal to about 99.9 percent of the theoreticaldensity wherein the theoretical density is based upon the compositionand amount of the specific starting materials. TABLE 10 SelectedPhysical Parameters for Sintered/HIP Alumina-Boron Carbide CompositionCA340-80B Young's Vickers Modulus Hardness K_(IC) (E & C) Example (GPa)(GPa) (MPa · m^(1/2)) 1 399.00 18.74 6.12 2 399.00 19.02 6.95 3 399.0017.80 6.90 Average 399.00 18.52 6.65

The density for Example CA340-80B is 3.731 grams per cubic centimeter(g/cc), which is equal to about 101 percent of the theoretical densitywherein the theoretical density is based upon the composition and amountof the specific starting materials.

Referring to the photomicrograph of the CA340-80B material, FIG. 6, thisoptical image shows the microstructure of the polished sample. Lookingat the photomicrograph (FIG. 6), applicants believe that four phases areshown in FIG. 6. What applicants believe to be the boron carbide (B₄C)phase is shown in FIG. 6 as a bright phase. In regard to the ytterbiumboride (YbB₆) phase, which applicants believe is present, while thisphase is visible under a scanning electron microscope, the resolution inthis optical photomicrograph (FIG. 6) is such that this phase isdifficult to distinguish. Applicants consider the balance of themicrostructure shown in FIG. 6 to comprise a matrix. Applicants believethat the matrix comprises an alumina (aluminum oxide) phase and analumina-based solid solution phase wherein the contrast in colordistinguishes between these phases. In this regard, what applicantsbelieve to be the alumina phase is shown in FIG. 6 as the phase of thematrix that is lighter (relative to the other matrix phase) in color,and what applicants believe to be the alumina-based solid solution phaseis shown in FIG. 6 as the phase of the matrix that is darker (relativeto the other matrix phase) in color. The dark areas in the opticalphotomicrograph (FIG. 6) appear to be locations at which the boroncarbide has been pulled out during sample preparation. The XRD analysisresults as presented in FIG. 7 verified the presence of alumina (Al₂O₃),boron carbide (B₄C) and ytterbium boride (YbB₆). TABLE 11 SelectedPhysical Parameters for Sintered/HIP Alumina-Boron Carbide CompositionCA340-80D Young's Vickers Modulus Hardness K_(IC) (E & C) Sample (GPa)(GPa) (MPa · m^(1/2)) 1 398.00 17.93 6.74 2 398.00 18.74 5.73 3 398.0018.46 6.29 Average 398.00 18.38 6.25

The density of Example CA340-80D is equal to 3.736 grams per cubiccentimeter, which is equal to about 101 percent of the theoreticaldensity wherein the theoretical density is based upon the compositionand amount of the specific starting materials.

Referring to the photomicrograph of the CA340-80D material, FIG. 8, thisoptical image shows the microstructure of the polished sample. Lookingat the photomicrograph (FIG. 8), applicants believe that four phases areshown in FIG. 8. What applicants believe to be the boron carbide (B₄C)phase is shown in FIG. 8 as a bright phase. In regard to the ytterbiumboride (YbB₆) phase, which applicants believe is present, while thisphase is visible under a scanning electron microscope, the resolution inthis optical photomicrograph (FIG. 8) is such that this phase isdifficult to distinguish. Applicants consider the balance of themicrostructure shown in FIG. 8 to comprise a matrix. Applicants believethat the matrix comprises an alumina (aluminum oxide) phase and analumina-based solid solution phase wherein the contrast in colordistinguishes between these phases. In this regard, what applicantsbelieve to be the alumina phase is shown in FIG. 8 as the phase of thematrix that is lighter (relative to the other matrix phase) in color,and what applicants believe to be the alumina-based solid solution phaseis shown in FIG. 8 as the phase of the matrix that is darker (relativeto the other matrix phase) in color. The dark areas in the opticalphotomicrograph (FIG. 8) appear to be locations at which the boroncarbide has been pulled out during sample preparation. The XRD analysisresults as presented in FIG. 9 verified the presence of alumina (Al₂O₃),boron carbide (B₄C) and ytterbium boride (YbB₆).

No amount of the initial Yb₂O₃ component was detectable via XRDanalysis. Hence, applicants believe that the ytterbia (Yb₂O₃) componenteither completely reacted with B₄C to form the ytterbium boride (YbB₆)detected via XRD analysis or reacted to such an extent where ytterbiumboride (YbB₆) was detectable via XRD analysis any remaining (if that wasthe case) and ytterbia was not detectable via XRD analysis.

Referring to the photomicrograph of the CA340-87 material, FIG. 10, thisoptical image shows the microstructure of the polished sample. Lookingat the photomicrograph (FIG. 10), applicants believe that four phasesare shown in FIG. 10. What applicants believe to be the boron carbide(B₄C) phase and the zirconium boride (ZrB₂) phase are each shown in FIG.10 as a bright phase. In this regard, the resolution in the opticalphotomicrograph is such that it is difficult to distinguish between theboron carbide phase and the zirconium boride phase. Applicants considerthe balance of the microstructure shown in FIG. 10 to comprise a matrix.Applicants believe that the matrix comprises an alumina (aluminum oxide)phase and an alumina-based solid solution phase wherein the contrast incolor distinguishes between these phases. In this regard, whatapplicants believe to be the alumina phase is shown in FIG. 10 as thephase of the matrix that is lighter (relative to the other matrix phase)in color, and what applicants believe to be the alumina-based solidsolution phase is shown in FIG. 10 as the phase of the matrix that isdarker (relative to the other matrix phase) in color. The dark areas inthe optical photomicrograph (FIG. 10) appear to be locations at whicheither the boron carbide or the zirconium boride has been pulled outduring sample preparation. The XRD analysis results as presented in FIG.11 shows the presence of alumina (Al₂O₃), boron carbide B₄C) andzirconium boride (ZrB₂). Applicants theorize that the chemical reactionbelow may have occurred during the sintering:4B₄C(s)+5ZrO₂(s)→5ZrB₂+4CO(g)+6BO(g)

No amount of initial zirconia (ZrO₂) component was detectable via XRDanalysis. Hence, applicants believe that the initial zirconia componenteither completely reacted with B₄C to form the ZrB₂ detected via the XRDanalysis or reacted to such an extent that the zirconium boride wasdetectable via XRD analysis and any remaining (if that was the case)zirconia was not detectable via XRD analysis. After the sinteringoperations, there were only three detectable phases that were detectedvia XRD analysis, and those were alumina (Al₂O₃), boron carbide (B₄C)and zirconium boride (ZrB₂). TABLE 12 Selected Physical Parameters forSintered/HIP Alumina-Boron Carbide Composition CA340-89A Young's VickersModulus Hardness K_(IC) (E & C) Sample (GPa) (GPa) (MPa · m^(1/2)) 1394.00 17.05 5.58 2 394.00 16.69 6.44 3 394.00 16.57 5.92 Average 394.0016.77 5.98The density of Example CA340-89A is equal to 3.703 grams per cubiccentimeter, which is equal to about 100 percent of the theoreticaldensity wherein the theoretical density is based upon the compositionand amount of the specific starting materials.

The XRD analysis results as presented in FIG. 12 shows the presence ofalumina (Al₂O₃), boron carbide (B₄C) and ytterbium boride (YbB₆). Themetallic aluminum was not detectable via XRD analysis. Applicantsbelieve that the metallic aluminum either completely reacted (or reactedto such an extent so as to not be detectable via XRD analysis) duringthe sintering by reacting with the thin B₂O₃ coating on the B₄Cparticles whereby the aluminum was converted to alumina while leavingboron which reacted with the carbon-rich environment to form B₄C.

No amount of the initial Yb₂O₃ component was detectable via XRDanalysis. Hence, applicants believe that the ytterbia component eithercompletely reacted with B₄C to form the YbB₆ detected via XRD analysisor reacted to such an extent where ytterbium boride was detectable viaXRD analysis and any remaining (if that was the case) ytterbia was notdetectable via XRD analysis. After the sintering operations, there wereonly three detectable phases that were detected via XRD analysis, andthose were alumina (Al₂O₃), boron carbide (B₄C) and zirconium boride(ZrB₂). TABLE 13 Selected Physical Parameters for Sintered/HIPAlumina-Boron Carbide Composition CA340-74A Young's Vickers ModulusHardness K_(IC) (E & C) Sample (GPa) (GPa) (MPa · m^(1/2)) 1 393.0015.79 7.32 2 393.00 16.80 6.74 3 393.00 16.57 6.92 Average 393.00 16.396.99

The density for Example CA340-74A is equal to 3.633 grams per cubiccentimeter, which is equal to about 99.9 percent of the theoreticaldensity wherein the theoretical density is based upon the compositionand amount of the specific starting materials. TABLE 14 SelectedPhysical Parameters for Sintered/HIP Alumina-Boron Carbide CompositionCA340-89B Young's Vickers Modulus Hardness K_(IC) (E & C) Sample (GPa)(GPa) (MPa · m^(1/2)) 1 390.00 17.29 6.50 2 390.00 17.93 6.29 3 390.0018.06 6.12 Average 390.00 17.76 6.30

The density for Example CA340-89B is equal to 3.695 grams per cubiccentimeter which is equal to about 99.9 percent of the theoreticaldensity wherein the theoretical density is based upon the compositionand amount of the specific starting materials. TABLE 15 SelectedPhysical Parameters for Sintered/HIP Alumina-Boron Carbide CompositionCA340-76A Young's Vickers Modulus Hardness K_(IC) (E & C) Sample (GPa)(GPa) (MPa · m^(1/2)) 1 368.00 18.19 5.74 2 368.00 17.29 6.66 3 368.0017.93 6.08 Average 368.00 17.80 6.16The density for Example CA340-76A is equal to 3.713 grams per cubiccentimeter, which is equal to about 101 percent of the theoreticaldensity wherein the theoretical density is based upon the compositionand amount of the specific starting materials.

The XRD analysis results as presented in FIG. 13 shows the presence ofalumina (Al₂O₃), boron carbide (B₄C) and ytterbium boride (YbB₆). Themetallic aluminum was not detectable via XRD analysis. Applicantsbelieve that the metallic aluminum either completely reacted (or reactedto such an extent so as to not be detectable via XRD analysis) duringthe sintering by reacting with the thin B₂O₃ coating on the B₄Cparticles whereby the aluminum was converted to alumina while leavingboron, which reacted with the carbon-rich environment to form B₄C.

No amount of the initial Yb₂O₃ component was detectable via XRDanalysis. Hence, applicants believe that the ytterbia component eithercompletely reacted with B₄C to form the YbB₆ detected via the XRDanalysis or reacted to such an extent where ytterbium boride wasdetectable via XRD analysis and any remaining (if that was the case)ytterbia was not detectable via XRD analysis. After the sinteringoperations, there were only three detectable phases that were detectedvia XRD analysis, and those were alumina (Al₂O₃), boron carbide (B₄C)and zirconium boride (ZrB₂).

It is not unusual for the boron carbide to have a thin coating of boronoxide (B₂O₃) thereon. Applicants are of the belief that the presence ofthe boron oxide coating has been detrimental to the densification of theceramic because the boron oxide inhibits the densification. Applicantsbelieve that it would be advantageous to be able to reduce the boronoxide content during sintering, and thus, protect the boron carbide fromoxidation loss and promote the densification.

In some of the specific examples, carbon black is present in ExampleCA340-80D (0.25 volume percent carbon black) and Example CA340-89B (0.75volume percent carbon black). Applicants theorize that since thesintering may have occurred in a carbon rich environment, the carbon mayhave reacted with the boron oxide coating to form B₄C according to thefollowing reaction:2B₂O₃(s)+7C(s)→B₄C(s)+6CO(g)It is thus apparent that the presence of the carbon black in thestarting powder mixture is beneficial to the reduction of the boronoxide and the densification.

A metallic aluminum component is present in Example CA340-80B (0.50volume percent metallic aluminum), Example CA340-89A (1.50 volumepercent metallic aluminum), Example CA340-74A (2.50 volume percentmetallic aluminum), and Example CA340-76A (2.50 volume percent metallicaluminum). For these examples that use metallic aluminum, applicantstheorize that the following reaction may have occurred during sinteringin a carbon-containing atmosphere (or if a carbon-containing componentwas present in the starting powder mixture):2B₂O₃(s)+4Al(s)+C(g/s)→Al₂O₃(s)+B₄C(s)

As can be appreciated from some of the examples, the thin film of boronoxide reacts upon sintering with the carbon or the aluminum to fromother compounds. In the case of aluminum, the resultant compounds couldbe alumina and boron carbide. In the case of carbon, the resultantcompounds could be boron carbide and carbon monoxide gas. For each case,there could occur the reduction of the boron oxide and the production ofboron carbide. Hence, the aluminum or carbon is present in an effectiveamount for the protection of the boron carbide from oxidation (loss) andto assist the densification.

Applicants contemplate that metallic additives other than aluminumshould function to protection the boron carbide from oxidation lossduring sintering. These metallic additives include magnesium and zinc.The carbon can be present in any one of a number of forms. Exemplaryforms are carbon black or an organic compound such as phenolic resin, aswell as a fugitive binder such as a wax.

In regard to the content in the starting powder mixture of the metallicadditive (e.g., aluminum, magnesium and/or zinc) and thecarbon-containing compound (e.g., carbon black or an organic compound),one preferred range is equal to between about 0.01 volume percent andabout 5.0 volume percent of the starting powder mixture. Anotherpreferred range for the content of the metallic additive and thecarbon-containing component is equal to between 0.05 volume percent andabout 3.0 volume percent of the starting powder mixture. Still anotherpreferred range for the content of the metallic additive and thecarbon-containing component is equal to between 0.50 volume percent andabout 2.0 volume percent of the starting powder mixture.

A zirconia (zirconium oxide) component is in Example CA340-87 (7.50volume percent zirconia). As set out above, applicants theorize that thezirconia may have reacted with the boron carbide to form zirconiumboride and carbon monoxide gas and boron oxide gas. In reference to thecontent of zirconia in the starting powder mixture, one preferred rangeis equal to between about 1 volume percent and about 15 volume percentof the starting powder mixture. Another preferred range of the zirconiais equal to between about 6 volume percent and about 10 volume percentof the starting powder mixture.

Table 16 is set out as a summary of the selected properties presented intables 9 through 15. TABLE 16 Average Values For Selected Properties inTables 9-15 Young's Vickers Modulus Hardness K_(IC) (E&C) Sample (GPa)(GPa) (MPa · m^(1/2)) CA-340-76 386.00 16.57 5.47 CA-340-89A 394.0016.77 5.98 CA-340-74A 393.00 16.39 6.99 CA-340-80D 398.00 18.38 6.25CA-340-80B 399.00 18.52 6.65 CA-340-89B 390.00 17.76 6.30 CA-340-76A368.00 17.80 6.16

Overall, it is apparent that applicants have invented a new and usefulceramic body that comprises alumina and a boron carbide irregular-shapedphase, and optionally, the sintering aid residue from a sintering aidcontained in the starting powder mixture. The ceramic body can be usedas a wear member, as well as an uncoated ceramic cutting insert or acoated ceramic cutting insert.

When used as a ceramic cutting insert, the ceramic substrate hasmaintained its wear resistance even at higher operating temperatures,especially those temperatures associated with higher cutting speeds(e.g., a speed equal to or greater than about 1500 sfm (457 smm), or ateven higher cutting speeds equal to or greater than about 2000 sfm (610smm)). The ceramic substrate has also been able to exhibit good chemicalresistance with respect to the workpiece material.

These improved properties demonstrate that overall the alumina-boroncarbide irregular-shaped phase ceramic cutting inserts of the inventionoutperform (as measured by mean tool life in the turning of ductile castiron) the conventional commercial ceramic cutting insert (a CVD coatedsilicon nitride cutting insert). This is especially true at highercutting speeds in the order of 2000 sfm (610 smm). This is also markedlyapparent when the sintering aid comprises a material like YAG or Yb₂O₃or La₂O₃ or Y₂O₃.

It is also apparent that applicants have invented a ceramic body thatcontains alumina and boron carbide, as well as a pressureless sinteringprocess for producing the ceramic body, wherein the pressurelesssintering process is not as expensive as hot pressing process.

It is also apparent that applicants have invented a ceramic body thatcontains alumina and boron carbide, as well as a process for producingthe ceramic body, wherein the process is able to fabricate parts with acomplicated or complex geometry.

All patents, patent applications, articles and other documentsidentified herein are hereby incorporated by reference herein. Otherembodiments of the invention may be apparent to those skilled in the artfrom a consideration of the specification or the practice of theinvention disclosed herein. It is intended that the specification andany examples set forth herein be considered as illustrative only, withthe true spirit and scope of the invention being indicated by thefollowing claims.

1. A ceramic metalcutting insert for chip forming machining made from astarting powder mixture, the ceramic insert body comprising: a substratehaving a rake surface and a flank surface wherein the rake surface andthe flank surface intersect to form a cutting edge; the substratecomprising between about 15 volume percent and about 35 volume percentof a boron carbide irregular-shaped phase and at least about 50 volumepercent alumina; and the substrate having a fracture toughness (K_(IC),18.5 Kg Load E&C) greater than or equal to about 4.5 MPa·m^(0.5).
 2. Theceramic metalcutting insert according to claim 1 wherein the substratehas a fracture toughness (K_(IC), 18.5 Kg Load E&C) greater than orequal to about 5.5 MPa·m^(0.5).
 3. The ceramic metalcutting insertaccording to claim 1 wherein the substrate has a Young's Modulus equalto or greater than about 300 GPa.
 4. The ceramic metalcutting insertaccording to claim 1 wherein the substrate has a Vicker's micro-hardnessequal to or greater than about 17 GPa.
 5. The ceramic metalcuttinginsert according to claim 1 wherein the substrate comprises betweenabout 15 volume percent and about 35 volume percent of the boron carbideirregular-shaped phase and between about 65 volume percent and about 85volume percent alumina.
 6. The ceramic metalcutting insert according toclaim 1 wherein the substrate comprises between about 20 volume percentand about 30 volume percent of the boron carbide irregular-shaped phaseand between about 70 volume percent and about 80 volume percent alumina.7. The ceramic metalcutting insert according to claim 1 wherein thesubstrate comprises about 25 volume percent of the boron carbideirregular-shaped phase and about 75 volume percent alumina.
 8. Theceramic metalcutting insert according to claim 1 wherein the substratefurther comprises residue from a sintering aid in the starting powdermixture and the sintering aid is selected from the group comprisingyttrium oxide, ytterbium oxide, yttrium aluminum garnet, lanthanumoxide, chromium oxide, and other rare earth oxides.
 9. The ceramicmetalcutting insert according to claim 1 wherein the substrate furtherincludes the constituents from one or more of the following additives inthe starting powder mixture: the oxides of hafnium and/or zirconium, andsilicon carbide whiskers.
 10. The ceramic metalcutting insert accordingto claim 1 further including a refractory coating on the substrate. 11.A process for making a ceramic body comprising the steps of: providing astarting powder mixture, the starting powder mixture comprises betweenabout 15 volume percent and about 35 volume percent boron carbide powderand at least about 50 volume percent alumina powder and no more thanabout 5 volume percent of a sintering aid; and consolidating the powdermixture at a temperature equal to between about 1400 degrees Centigradeand 1850 degrees Centigrade to achieve a ceramic with a density equal togreater than 99 percent of theoretical density.
 12. The processaccording to claim 11 wherein the consolidating conditions furthercomprise a hot pressing pressure equal to between about 30 MPa and about40 MPa.
 13. The process according to claim 11 wherein the consolidatingconditions further comprise a hot pressing duration equal to betweenabout 30 minutes and about 90 minutes.
 14. The process according toclaim 11 wherein the ceramic body has a fracture toughness (K_(IC), 18.5Kg Load E&C) greater than or equal to about 4.5 MPa·m^(0.5).
 15. Theprocess according to claim 11 wherein the ceramic body has a Young'sModulus greater than or equal to about 300 GPa.
 16. The processaccording to claim 11 wherein: the starting powder mixture comprisesbetween about 20 volume percent and about 30 volume percent of the boroncarbide powder and between about 70 volume percent and about 80 volumepercent of the alumina powder.
 17. The process according to claim 11further including the step of applying a coating to the ceramic body.18. A ceramic body comprising: between about 15 volume percent and about35 volume percent of a boron carbide irregular-shaped phase, and atleast about 50 volume percent alumina, and the ceramic body has afracture toughness (K_(IC), 18.5 Kg Load E&C) greater than or equal toabout 4.5 MPa·m^(0.5).
 19. The ceramic body according to claim 18wherein the body further includes the residue from a sintering aid. 20.A method of machining a workpiece comprising the steps of: providing aworkpiece; providing a ceramic cutting insert having a rake surface anda flank surface wherein the rake surface and the flank surface intersectto form a cutting edge and the ceramic cutting insert having a substratethat comprises between about 15 volume percent and about 35 volumepercent of a boron carbide phase and at least about 50 volume percentalumina and has a fracture toughness (K_(IC), 18.5 Kg Load E&C) greaterthan or equal to about 4.5 MPa·m^(0.5); causing relative rotationalmovement between the workpiece and the ceramic cutting insert whereinthe surface speed of the relative rotational movement is equal to orgreater than about 457 surface meters per minute; and bringing theceramic cutting insert and the workpiece into contact with each other soas to remove material from the workpiece.
 21. The method of claim 20wherein the surface speed of the relative rotational movement is equalto or greater than about 610 surface meters per minute.
 22. The methodof claim 20 wherein the workpiece material is cast iron.
 23. The methodof claim 20 wherein the workpiece material is ductile cast iron.
 24. Aceramic metalcutting insert for chip forming machining made from astarting powder mixture, the ceramic insert body comprising: a substratehaving a rake surface and a flank surface wherein the rake surface andthe flank surface intersect to form a cutting edge; the substratecomprising between about 15 volume percent and about 50 volume percentof a boron carbide irregular-shaped phase and at least about 50 volumepercent alumina, and the substrate further comprising at least one ofthe following components (a) through (b) wherein the sum of the amountsof components (a) and (b) is equal to an effective amount to achievefull densification and wherein components (a) and (b) comprise: (a) ametal component including one or more of aluminum, magnesium and zinc;or (b) a reduction component containing carbon.
 25. The ceramic bodyaccording to claim 24 wherein the substrate further including ytterbiumboride.
 26. The ceramic body according to claim 24 wherein thecontribution to the substrate from components (a) and (b) consistsessentially of aluminum.
 27. The ceramic body according to claim 24wherein the contribution to the substrate from components (a) and (b)consists essentially of magnesium.
 28. The ceramic body according toclaim 24 wherein the contribution to the substrate from components (a)and (b) consists essentially of zinc.
 29. The ceramic body according toclaim 24 wherein the contribution to the substrate from components (a)and (b) consists essentially of carbon.
 30. The ceramic body accordingto claim 24 wherein the substrate contains between about 15 volumepercent and about 35 volume percent boron carbide, and at least about 50volume percent alumina.
 31. The ceramic body according to claim 24wherein the substrate contains between about 20 volume percent and about30 volume percent boron carbide, and at least about 50 volume percentalumina.
 32. The ceramic body according to claim 24 wherein theeffective amount of components (a) and (b) is equal to an amount betweenabout 0.01 volume percent and about 5.0 volume percent.
 33. The ceramicbody according to claim 24 wherein the effective amount of components(a) and (b) is equal to an amount between about 0.05 volume percent andabout 3.0 volume percent.
 34. The ceramic body according to claim 24wherein the sum of the amounts of components (a) and (b) is equal to anamount between about 0.50 volume percent and about 2.0 volume percent.35. The ceramic body according to claim 24 wherein the substrate havinga fracture toughness (K_(IC), 18.5 Kg Load E&C) greater than or equal toabout 4.0 MPa·m^(0.5).
 36. The ceramic body according to claim 24wherein the substrate having a fracture toughness (K_(IC), 18.5 Kg LoadE&C) greater than or equal to about 5.0 MPa·m^(0.5).
 37. The ceramicbody according to claim 24 wherein the substrate having a densitygreater than or equal to about 99 percent of the theoretical density.38. The ceramic body according to claim 24 wherein the substrate furthercontains the residue from one or more sintering aids selected from thegroup comprising yttrium oxide, ytterbium oxide, yttrium aluminumgarnet, lanthanum oxide, chromium oxide, and other rare earth oxides.39. A ceramic metalcutting insert for chip forming machining made from astarting powder mixture, the ceramic insert body comprising: a substratehaving a rake surface and a flank surface wherein the rake surface andthe flank surface intersect to form a cutting edge; the substratecomprising between about 15 volume percent and about 50 volume percentof a boron carbide irregular-shaped phase and at least about 50 volumepercent alumina, and the substrate further comprising at least one ofthe following components (a) and (6) and (c) wherein the sum of theamounts of components (a) and (b) and (c) is equal to an effectiveamount to achieve full densification and wherein components (a) and (b)and (c) comprise: (a) a metal component including one or more ofaluminum; (b) a reduction component containing carbon; or (c) an oxideincluding one or more of zirconium oxide and hafnium oxide.
 40. Theceramic insert according to claim 39 wherein the substrate having afracture toughness (K_(IC), 18.5 Kg Load E&C) greater than or equal toabout 4.0 MPa·m^(0.5).
 41. The ceramic insert according to claim 39wherein the substrate having a fracture toughness (K_(IC), 18.5 Kg LoadE&C) greater than or equal to about 5.0 MPa·m^(0.5).
 42. The ceramicinsert according to claim 39 wherein the contribution to the substratefrom components (a) and (b) and (c) consists essentially of alumina,boron carbide and one or more of zirconium carbide and hafnium carbide.43. The ceramic insert according to claim 39 wherein the substratecomprising between about 35 volume percent and 45 volume percent of aboron carbide irregularly-shaped phase and at least 50 volume percentalumina.
 44. The ceramic insert according to claim 39 wherein the sum ofthe amounts of components (a) and (b) and (c) equals between about 1volume percent and about 15 volume percent.
 45. The ceramic insertaccording to claim 39 wherein the sum of the amounts of components (a)and (b) and (c) equals between about 6 volume percent and about 10volume percent.
 46. The ceramic body according to claim 39 wherein thesubstrate further including ytterbium boride.
 47. The ceramic bodyaccording to claim 39 wherein the substrate contains between about 15volume percent and about 35 volume percent boron carbide, and at leastabout 50 volume percent alumina.
 48. The ceramic body according to claim39 wherein the substrate contains between about 20 volume percent andabout 30 volume percent boron carbide, and at least about 50 volumepercent alumina.
 49. The ceramic body according to claim 39 wherein thesubstrate having a density greater than or equal to 99 percent of thetheoretic density.
 50. The ceramic body according to claim 39 whereinthe substrate further contains the residue from one or more sinteringaids selected from the group comprising yttrium oxide, ytterbium oxide,yttrium aluminum garnet, lanthanum oxide, chromium oxide, and other rareearth oxides.
 51. A method of making a ceramic body comprising the stepsof: providing a starting powder mixture comprising: between about 15volume percent and about 50 volume percent of a boron carbideirregular-shaped phase and at least about 50 volume percent alumina, andthe substrate further comprising at least one of the followingcomponents (a) through (b) wherein the sum of the amounts of components(a) and (b) is equal to an effective amount to achieve fulldensification and wherein components (a) and (b) comprise: (a) a metalcomponent including one or more of aluminum, magnesium and zinc; and (b)a reduction component containing carbon; and forming a green compactfrom the starting powder mixture; sintering the green compact to fulldensity so as to form the ceramic body.
 52. The method according toclaim 51 wherein the starting powder mixture further contains one ormore sintering aids selected from the group comprising yttrium oxide,ytterbium oxide, yttrium aluminum garnet, lanthanum oxide, chromiumoxide, and other rare earth oxides.
 53. The method according to claim 51wherein the sintering to full density step comprises a pressurelesssintering step followed by a hot isostatic pressing step.
 54. The methodaccording to claim 51 wherein the pressureless sintering occurs at atemperature between about 1500° C. and about 1850° C. and for a durationequal to between about 15 minutes and about 180 minutes; and the hotisostatic pressing step occurs at a temperature equal to between about1400° C. and about 1725° C. and at a pressure equal to or greater thanabout 3.4 MPa and for a duration equal to between about 15 minutes andabout 120 minutes.
 55. The method according to claim 51 wherein thepressureless sintering occurs at a temperature between about 1600° C.and about 1750° C. and for a duration equal to between about 30 minutesand about 60 minutes, and wherein the hot isostatic pressing step occursat a temperature equal to between about 1600° C. and about 1700° C. andat a pressure equal to or greater than about 50 MPa and for a durationequal to between about 30 minutes and about 60 minutes.
 56. The methodaccording to claim 51 wherein the effective amount of components (a) and(b) is equal to between about 0.01 volume percent and about 5.0 volumepercent of the starting powder mixture.
 57. The method according toclaim 51 wherein the ceramic body has a density greater than or equal toabout 99 percent of the theoretical density.
 58. A method of making aceramic body comprising the steps of: providing a starting powdermixture comprising: between about 15 volume percent and about 50 volumepercent of a boron carbide irregular-shaped phase and at least about 50volume percent alumina, and the substrate further comprising at leastone of the following components (a) through (c) wherein the sum of theamounts of components (a) and (b) and (c) is equal to an effectiveamount to achieve full densification; and wherein components (a) and (b)and (c) comprise: (a) a metal component including one or more ofaluminum; (b) a reduction component containing carbon; or (c) an oxideincluding one or more of zirconium oxide and hafnium oxide; and forminga green compact from the starting powder mixture: sintering the greencompact to full density so as to form the ceramic body.
 59. The methodaccording to claim 58 wherein the starting powder mixture furthercontains one or more sintering aids selected from the group comprisingyttrium oxide, ytterbium oxide, yttrium aluminum garnet, lanthanumoxide, chromium oxide, and other rare earth oxides.
 60. The methodaccording to claim 58 wherein the pressureless sintering occurs at atemperature between about 1500° C. and about 1850° C. and for a durationequal to between about 15 minutes and about 180 minutes; and the hotisostatic pressing step occurs at a temperature equal to between about1400° C. and about 1725° C. and at a pressure equal to or greater thanabout 3.4 MPa and for a duration equal to between about 15 minutes andabout 120 minutes.
 61. The method according to claim 58 wherein thepressureless sintering occurs at a temperature between about 1600° C.and about 1750° C. and for a duration equal to between about 30 minutesand about 60 minutes, and wherein the hot isostatic pressing step occursat a temperature equal to between about 1600° C. and about 1700° C. andat a pressure equal to or greater than about 50 MPa and for a durationequal to between about 30 minutes and about 60 minutes.
 62. The methodaccording to claim 58 wherein the effective amount of components (a) and(b) and (c) is equal to between about 1 volume percent and about 15volume percent of the starting powder mixture.
 63. The method accordingto claim 58 wherein the ceramic body has a density greater than or equalto about 99 percent of the theoretical density.